registry_get_text(key, value)
Get a registry value. If not present, set and return the default.

DECLARE
v_value text;
BEGIN
select reg_text into v_value from sl_registry
where reg_key = p_key;
if not found then
v_value = p_default;
if p_default notnull then
perform registry_set_text(p_key, p_default);
end if;
else
if v_value is null then
raise exception 'Slony-I: registry key % is not a text value',
p_key;
end if;
end if;
return v_value;
END;